Molecular dynamics simulations of thin film growth by Ge nanocluster deposition

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Title: Molecular dynamics simulations of thin film growth by Ge nanocluster deposition
Author: Majorin, Peter
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2020
Language: eng
Thesis level: master's thesis
Discipline: Fysiikka
Abstract: The ionized cluster beam (ICB) technique for thin film deposition can produce thin films of superior quality compared to conventional methods. This technique relies on depositing nanoclusters as building blocks for the thin film. To gain a deeper insight into the thin film formation process, molecular dynamics simulations can be used. In this thesis, Ge nanocluster deposition on a (001) Ge substrate at 300 K was studied with molecular dynamics. The Stillinger-Weber potential was used in all of the simulations, and the nanoclusters had a kinetic energy of 5 meV - 10 eV. The formed Ge thin films were analyzed for porosity, density, crystallinity, and coordination. The density analysis showed that porous films were formed with lower deposition energies of 5 meV - 0.5 eV, and dense amorphous films with higher deposition energies of 1.0 eV - 10 eV. The radial distribution function and a crystalline atom count showed that the degree of crystallinity of the thin film is gradually lost with increasing deposition energies. Almost no epitaxial growth of the deposited thin film was detected. In addition, small amounts of hexagonal Ge was detected on the surfaces of the deposited nanoclusters with deposition energies of 5 meV - 100 meV. By heating a free nanocluster we showed that hexagonal Ge is formed on nanocluster surfaces independently of the cluster-surface interactions. With higher deposition energies of 0.5 eV - 10 eV all nanocrystallinity was lost, and a density analysis showed up to 10% higher density than bulk crystalline density. The higher density was also accompanied by a higher mean coordination number of around 4.5. This unusually dense amorphous material has not been reported elsewhere, so it is likely that the parametrization of the Stillinger-Weber potential overestimates the coordination and density of the formed thin film. The fast heat dissipation used in the simulations may also be a part of this problem, and it is not known how well it matches experimental settings.
Subject: germanium
molecular dynamics
thin films

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